The present invention relates to a magnetic resonance apparatus for detecting the chemical shift information of low-sensitive .sup.13 C and .sup.15 N from a high-sensitive .sup.1 H with a high sensitivity level.
Many of an MRI (Magnetic Resonance Imaging) are the technique of non-invasively imaging the distribution of water in a living body by detecting the .sup.1 H of the water molecule. The main information obtained with an image of a present-state water distribution is of a configuration type.
Attention has been recently paid to an MRS (Magnetic Resonance Spectroscopy) for obtaining the atomic nucleus information other than that of the .sup.1 H of the water, so as to non-invasively obtain the metabolic information in the living tissue through the detection of, for example, a metabolic product, .sup.1 H, .sup.13 C or .sup.31 P. Of these, particular attention has been recently focused on the .sup.13 C-MRS. Since the ratio of .sup.13 C present in nature is as low as 1.1%, it is possible to highly accurately trace the manner of a metabolism after the dosing of a .sup.13 C labeled substance and to obtain the metabolic information different from .sup.1 H and .sup.31 P.
In the .sup.13 C-MRS, however, these has been the low S/N ratio problem. In order to overcome this problem, the .sup.1 H detected method has been developed according to which it is possible to observe the .sup.13 C, with high sensitivity, from the .sup.1 H which is magnetically coupled (J-coupled) to .sup.13 C. As one example, there is a pulse sequence of an HSQC in FIG. 1. This is a pulse sequence of applying first and second RF pulses to the .sup.1 H, then third and fourth RF pulses to the .sup.13 C and then a fifth RF pulse to the .sup.1 H. From the first and second RF pulses to the .sup.1 H and third RF pulse to the .sup.13 C a polarization transfer (first polarization transfer) occurs from the .sup.1 H to the .sup.13 C.
At a subsequent "evalution" time t1, the magnetization evolves with .sup.13 C chemical shift and, by the fourth RF pulse to the .sup.13 C and fifth RF pulse to the .sup.1 H, a polarization transfer (second polarization transfer) again occurs from the .sup.13 C to the .sup.1 H. By doing so it is possible to observe the MR signal of the .sup.1 H experiencing such polarization transfer on the .sup.1 H side.
It is not until the .sup.13 C-MRS can provide the clinically useful information that such .sup.13 C-MRS can measure those spectra on a spatially adequately localized volume. Therefore, various methods are offered in this connection.
As representative examples, there are a method for applying, to the .sup.13 C-MRSI (Magnetic Resonance Spectroscopic Imaging) method for imparting a signal to the spatial information with the use of a phase encoding gradient magnetic field pulse, a method (J. Magn. Reson. Vol. 80, p.162 to p.167, 1998) of Lizann Bolinger, et al., applied to an ISIS (Image Selected In vivo Spectroscopy), and so on.
In the acquision of a signal from the N region for instance, it is necessary for these methods to run the pulse sequence N times. For the MRSI with the use of the encoding gradient field pulse for instance, it is necessary to obtain signals N times by varying the time integral of the intensity of the encoding gradient magnetic field pulses. Further, in order to obtain a spatial two-dimensional .sup.13 C signal of a matrix size Nx.times.Ny, it is required that the scan be performed Nx.times.Ny times. That is, a longer scan time is required, thus posing the problem.
This problem provides an extra problem when the two-dimensional spectroscopy of the .sup.1 H and .sup.13 C is done with the HSQC for instance. With the number of steps in a frequency encoding direction done Nc times, it is necessary to obtain a signal Nx.times.Ny.times.Nc times at a spatial two-dimension mapping. Let it be supposed that, for example, the repetition time TR is one second; the Nc, 128 and Ny, 8. Then it takes 136 minutes to effect checking once. For the .sup.13 C-MRS it is necessary to get the time variation of the spectrum after the dosing of the labeled reagent. In this case, it takes 136 minutes to make one checking and it is not possible at all to obtain a time variation involved.
For the MRI for imaging a spatial distribution of the .sup.1 H in the water, such a longer scan time problem can be solved by a multi-slice imaging method.
FIG. 2 shows a pulse sequence when the multi-slice imaging method is applied to the spin echo method. In the imaging of slice regions SL1 and SL2 shown in FIG. 3, a spin echo is generated from those spins in the first slice area SL1 at a first excited pulse (90.degree. pulse) and refocus pulse (180.degree. pulse). Since, at this time, those spins outside the slice area SL1 are not excited, it is possible to create a spin echo from those spins in the slice area immediately after the data collection of the first slice area SL1 is completed. This method is of advantage in that the number of scans is not increased because data acquisition can be made from a plurality of areas in a one repetition time TR.
For the MRI, the multi-slice imaging method is very effective. It is difficult to apply this to the so-called the .sup.1 H detected method for collecting the .sup.13 C spectra with .sup.1 H sensitivity as in the HSQC in FIG. 1.
FIG. 4 shows the case of applying the multi-slice method to the HSQC sequence. It is required, in this case, that all the RF pulses be made a slice select pulse G.sub.slice. That is, first a signal of a slice region SL1 is observed and, in this case, a thermal equilibrium state prevails at other than the slice area SL1. Then another slice area SL2 is selected with the RF pulse and it is possible to observe a signal of a slice area SL2. By this method, signals from a plurality of regions can be collected by one scanning.
Since the spin repetitively experiences the slice select pulses, the incompleteness of the slice profile characteristic possessed by the slice select pulse produces an adverse cumulative effect. It is, therefore, unavoidable that there occurs an increase in signal loss.
Further, the .sup.13 C RF pulse is used as a slice select excitation pulse but, since there is a broader resonance bandpath for the .sup.13 C chemical shift, there occurs a temporal shift resulting from the pulse excitation, by the slice selection, over a broader range.
It has, therefore, been difficult to apply the conventional multi-slice method to the .sup.13 C-MRS.